The invention generally relates to the storage of gas in solids. This type of storage generally stores gas at storage pressures less than those found in the case of purely gaseous storage. Applications of this type of storage are diverse and relate for example to the use of hydrogen in a fuel cell intended for the production of electricity, or the use of ammonia in applications for reduction of nitrogen oxides NOx by selective catalytic reduction (SCR), especially for reduction of pollutant emissions by the internal combustion engines, diesel engines in particular. The invention relates to an ammonia storage structure especially for selective catalytic reduction of nitrogen oxides in the exhaust gas of combustion vehicles, comprising at least one storage material in which the ammonia can be stored. The invention also relates to systems comprising such a structure.
The reduction of pollutant emissions linked to transport has been the object of development for nearly thirty years. The progressive rise in the severity of emission limits for the four regulated pollutants (CO, HC, NOx, particles) has significantly improved the quality of air especially in large conurbations.
The growing use of cars means continuing efforts to further reduce these pollutant emissions. A decrease in tolerance relative to European emission thresholds is expected in 2014 in terms of steps for the introduction of the standard Euro6. Such measures aim to reduce local pollution. In this context, it is preferable to reduce nitrogen oxides (NOx) in a poor mixture, that is, a mixture comprising oxygen in excess.
Also, fuel consumption, in direct connection with CO2 emissions, has been propelled in a few years to a major preoccupation level of the automobile. So a ruling has been put in place at the European level from 2012 to limit CO2 emissions of particular vehicles. It is already accepted that this limit will be lowered regularly over decades to come. This double problem: reduction of local pollution (NOx), and reduction in fuel consumption (CO2), is particularly restrictive for diesel engines the combustion of which in poor mixture is accompanied by NOx emissions difficult to treat.
In this context, SCR post-processing technology (“selective catalytic reduction”) is used as much for particular vehicles as for vehicles related to transport of merchandise. An SCR system generally reduces nitrogen oxides NOx by selective catalytic reduction. It is possible to operate an engine optimally at yield at a cost of substantial NOx emissions, these NOx emissions then being treated in the exhaust by an SCR system enabling NOx reduction with considerable efficiency. To put such SCR technology in place it is necessary to place on board the vehicle a reducing agent for reduction of nitrogen oxides.
The SCR system currently used by heavy-goods vehicles uses urea in an aqueous solution as reducing agent. Injected into the exhaust, urea decomposes by the effect of temperature of the exhaust gas into ammonia (NH3) and allows reduction of NOx on a specific catalyst. An aqueous solution of urea standardised for operation of systems currently in SCR series is referenced AUS32 (the commercial name in Europe is Adblue®). This method is subject to some limitations. It exhibits limited cold efficiency (engine not yet warm). Such a situation occurs in several cases, especially for city buses.
Also, the urea tank has substantial mass and volume, typically 15 to 30 L for a particular vehicle, 40 to 80 L for a heavy-goods vehicle. Such bulk causes complexity of integration in the vehicle which is all the greater since the vehicle is small. This results in high decontamination costs, and surplus mass to the detriment of fuel consumption of the vehicle and therefore of CO2 emissions.
Alternative storage methods have therefore been planned to try to overcome these limitations. The option consisting of storing gas under pressure in an empty tank also has disadvantages, especially in terms of compactness and operation safety. This applies especially to the storage of gaseous ammonia.
Another method consists of storing gas inside a storage material, in which the gas is absorbed. This storage material, for example salt, is arranged in a storage enclosure. The storage of gas (typically ammonia which is the example to be developed here, but this principle applies to the storage of other gases) is done in salt by formation of a chemical complex of ammoniate type.
The following paragraph gives more detail on the chemical process of ammonia sorption in material such as salt. In a storage structure, a powdered salt is selected from alkaline earth chlorides as storage material. In particular, powdered salt can be selected from the following compounds: SrCl2, MgCl2, BaCl2, CaCl2, NaCl2.
The storage of ammonia in such a storage material is based on reversible solid-gas reaction of type:<Solid A>+(Gas)⇄<Solid B>The ammonia forms coordination complexes with the alkaline earth chlorides, also called ammoniates. The skilled person knows this phenomenon.
For example, reactions of ammonia with the strontium chloride are:SrCl2(s)+NH3(g)⇄Sr(NH3)Cl2(s)Sr(NH3)Cl2(s)+7NH3(g)⇄Sr(NH3)8Cl2(s)Similarly, the unique reaction of ammonia with barium chloride is:BaCl2(s)+8NH3(g)⇄Ba(NH3)8Cl2(s)
The chemical absorption of the ammonia binder by the absorbent SrCl2 and BaCl2 causes, between the solid and the gas, a transfer of electrons which occurs via chemicals bonds between NH3 and the external layer of atoms of SrCl2 and BaCl2. The penetration of gas into the structure of the solid occurs in its entire mass by a diffusion method. This reaction is reversible, absorption being exothermal and desorption endothermal.
This type of storage has advantages. Storage in salt in fact significantly reduces mass and volume of the storage tank. It also provides a benefit in terms of CO2 balance due to the decrease in reducer mass to be embedded for given autonomy of ammonia. Relative to the storage of urea in aqueous solution, in fact the extra quantity of water provided to dilute the urea in the classic configuration of the SCR is saved. Also, this type of storage uses cold NOx absorption with higher efficiency. This type of storage also enables a reduction in manufacturing costs since the supply system and injection of the ammonia can be simplified.
The focus throughout this text will be on this type of storage. To limit the bulk of the storage enclosure, car manufacturers use filling or replacement of the storage enclosure, for example during maintenance, at the time of emptying, or during filling of the fuel tank. According to currently held hypotheses, the quantity of ammonia embedded on board a particular vehicle will be of the order of 6 kg for an equivalent of 16 Liters of a solution of urea of type AUS32, which ensures autonomy of the particular vehicle between two emptying intervals of the vehicle. To allow supply of an SCR system with ammonia, a heating element is provided, electrical or via a coolant fluid for example, controlled so as to release in doses in each condition of use, ammonia intended for processing of nitrogen oxides.
In an envisaged embodiment, once the storage enclosure (for example a cartridge—these two terms ‘enclosures’ and ‘cartridge’ can be used in this text) is empty, it is replaced by a full cartridge, for example during vehicle maintenance, the empty cartridge being sent to a filling station. A cartridge could undergo from ten to fifteen emptying/filling cycles. According to the strategies of manufacturers, the frequency of exchange of storage enclosures and their modalities of exchange could be modulated.
The storage of ammonia in the form of absorbed gas therefore has advantages relative to an aqueous solution of Adblue (gain in volume, increased cold efficiency, more compactness of the mixing area with the exhaust gas, . . . ). The aim of the invention is to further improve known SCR systems.
In particular, different aspects of the invention aim to contribute a solution to at least one of the following problems:                Overcoming to some degree the contradiction inherent to known devices, between the search for minimal gas pressure in the storage enclosure, and minimisation of power (typically of electrical origin) necessary to release the stored gaseous ammonia,        The difficulty in gauging the level of gas, which is stored in a solid matrix. In this respect, planning of the exchange of empty cartridges by full cartridges would be considerably facilitated if it were possible to gauge the level of said cartridges over time,        The heterogeneity set progressively in the cartridge by the process of emptying said cartridges over the life of the system. This progressive emptying will in fact induce progressive heterogeneity in the storage matrix, which could cause evolution in system performance. In due course this can also cause a change in the inherent characteristics of this matrix, and consequently durability problems.To provide at least one of these solutions, the invention proposes an ammonia storage structure especially for selective catalytic reduction of nitrogen oxides in the exhaust gas of combustion vehicles, comprising at least one storage material in which ammonia can be stored, characterized in that it comprises at least two separate storage parts, each storage part containing storage material, the storage materials of the different storage parts not all being identical.        
Advantageous, but non-limiting, aspects of such a structure are the following:                the different storage materials have different sorption enthalpies,        the different storage materials have different porosities, or different distributions of sizes of pores,        the different storage materials have different thermal conductivities,        at least some of the storage materials are in powdered form,        at least some of the storage materials are in the form of rigid elements,        the materials are selected from alkaline earth chlorides, in particular in the form of salt of SrCl2, MgCl2, BaCl2, CaCl2, or NaCl2,        the storage parts are arranged adjacent to each other and means are provided to enable circulation of gaseous ammonia between two adjacent storage parts,        the structure comprises means enabling circulation of gaseous ammonia between two adjacent storage parts,        said means enabling the circulation of gaseous ammonia between two adjacent storage parts are controlled to control circulation of gaseous ammonia between two adjacent storage parts,        said means enabling circulation of gaseous ammonia between two adjacent storage parts are controlled obturating means,        to enable circulation of gaseous ammonia between two adjacent storage parts the structure comprises a gas transport device such as a conduit or a diffuser,        to enable circulation of gaseous ammonia between two adjacent storage parts, the structure comprises an intermediate element fitted with holes or whereof the porosity allows diffusion of the gaseous ammonia,        at least one storage part contains a heating element,        the heating element is an electrical resistor.The invention also relates to an ammonia storage and destocking system of a vehicle comprising a storage enclosure, the storage enclosure comprising a storage structure according to one of the aspects hereinabove.        
The invention also relates to a control method of a storage structure of an ammonia storage and destocking system as described previously, the method comprising:                a first control step of the heating element of the first storage part so as to release ammonia stored in the first storage part, and        a second monitoring step of the quantity of ammonia released by the first storage part and/or of the quantity of ammonia stored in the first storage part.The variation in the quantity of ammonia of the first storage part can be monitored independently of the second storage part, in particular since ammonia is stored in the second storage part without the second storage part releasing stored ammonia.        
Advantageous, but non-limiting, aspects of such a method are the following:                in response to an indication by the sensor of the first storage part that the quantity of stored ammonia is less than a given threshold, a third step for release of the ammonia stored in the second storage part;        the third step comprises controlling a heating element of the second storage part so as to release ammonia stored in the second storage part;        the third step comprises opening control of controlled obturating means separating the first storage part from the second storage part;        a fourth step for monitoring the quantity of ammonia released by the second storage part and/or the quantity of ammonia stored in the second storage part.        
The invention also relates to a system of selective catalytic reduction for internal combustion engine exhaust gas, comprising an ammonia storage system such as mentioned hereinabove and a module for injection ammonia into the exhaust gas. According to an advantageous, but non-limiting aspect, the system of selective catalytic reduction for exhaust gas of an internal combustion engine comprises control means configured to use a control method such as described previously.